Spermacoce glabra Michx.
Family - Rubiaceae
Stem - Ascending to erect, to 60 cm, squarish in cross section, glabrous.
Stem and leaves.
Leaves - Opposite, sessile, simple, entire, to 8 cm long, narrowly ellpitic, glabrous.
Stipules - Interpetiolar, fused to leaf bases, bearing 5-7 apical bristles, these ranging in length to 6 mm long.
Inflorescence - Inflorescences axial at upper stem nodes, dense sessile clusters.
Flowers - Calyces deeply 4-lobed, with triangular lobes to 2 mm long, glabrous. Corollas 4-lobed, to 4 mm long, externally glabrous, internally bearded, white. Stamens 4, adnate to corolla. Stigmas 2. Ovary inferior, 2-locular with 1 ovule per locule.
Fruits - Achenelike, indehiscent, 3-4 mm long, obconic and somewhat flattened laterally, the surface smooth, leathery to stiffly papery.
Flowering - June - October.
Habitat - Streambanks, pond margins, other moist areas.
Origin - Native to the U.S.
Other info. - This common denizen of moist riverbanks is similar in appearance to the bugleweeds (Lycopus spp). It is easily distinguished by its interpetiolar stipules, which immediately demonstrate its family affiliation to be Rubiaceae rather than the bugleweeds' Lamiaceae. In Missouri the plant is found almost exclusively in the southern half of the state.
Photographs taken at Riverlands Migratory Bird Sanctuary, St. Charles County, MO, 7-21-2013, and at Dresser Island Conservation Area, St. Charles County, MO, 7-22-2013 (SRTurner).